This invention relates generally to calcined clay products, and more specifically relates to an anhydrous white kaolin clay pigment and method of manufacture of same. The pigment is particularly useful as a filler in paper products, but also finds use in other applications, such as in coating of papers. In accordance with the invention a method is disclosed for effectively reducing the abrasion value of this pigment.
In the course of manufacturing paper and similar products, including paperboard and the like, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. In the absence of such "fillers", the resultant paper can have a relatively poor texture due to discontinuities in the fibrous web. The said fillers are also important in improving the printing qualities of the paper, i.e., by improving the surface characteristics of same. The use of appropriate such fillers, further, vastly improves the opacity and the brightness of a paper sheet of a given weight.
A number of inorganic materials have long been known to be effective for many of the aforementioned purposes. Among the best of these materials is titanium dioxide, which can be incorporated into the paper in the form of anatase or of rutile. Titanium dioxide, however, is among the most expensive materials which are so usable. Thus, despite the effectiveness of such material as a filler, its use is limited, and satisfactory replacements have been much sought after.
Among the materials which have thus found increasing acceptance as paper fillers are calcined kaolin clays. Materials of this type are generally prepared by calcining a crude kaolin clay, which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product. Reference may be usefully had to Proctor, U.S. Pat. No. 3,014,836, and to Fanselow, U.S. Pat. No. 3,586,523, and U.S. Pat. No. 4,381,948, which disclosures are representative of the prior art pertinent to the present invention.
Those properties which render a calcined clay pigment particularly valuable for use as a filler are also well known. These include a low abrasion value, and high brightness and opacifying characteristics. The low abrasion is significant in order to ensure that the resultant paper product may be manufactured and processed using conventional machinery without damaging same. The brightness and opacifying characteristics are important in producing an acceptable paper sheet, one which incorporates whiteness, high opacity, good printability and light weight.
Both the brightness characteristics of a given kaolin clay and the opacifying properties of same, when incorporated as a filler in paper, may be quantitatively related to a property of the filler identified as the "scattering coefficient S". The said parameter, i.e. the scattering coefficient S of a given pigment, is a property well known and extensively utilized in the paper technology art, and has been the subject of numerous technical papers and the like. The early exposition of such measurements was made by Kubelka and Munk, and is reported in Z. Tech Physik 12:539(1931). Further citations to the applicable measurement techniques and detailed definitions of the said scattering coefficient are set forth at numerous places in the patent and technical literature Reference may usefully be had in this connection e.g., to U.S. Pat. Nos. 4,026,726, and 4,028,173. In addition to the citations set forth in these patents, reference may further be had to Pulp and Paper Science Technology Vol. II "Paper", Chapter 3, by H. C. Schwalbe (McGraw-Hill Book Company, N.Y.).